This invention relates generally to a current limiting circuit breaker and more particularly to a stationary contact assembly for a current limiting circuit breaker.
The interrupting rating of a circuit breaker can be economically increased by using a current limiting set of contacts which may be added onto an existing breaker or integrally formed in a breaker. The current limiting contacts limit the amount of current available to the breaker to a preselected maximum amount which enables the breaker to be designed to interrupt that maximum amount rather than an unlimited amount from a source capable of generating extremely high short circuit currents. Typically, current limiting circuit breakers operate on an electromagnetic repulsion principle wherein as the magnitude of the fault current increases to a preselected magnitude, the magnetic forces build which tend to repel the movable contact away from the stationary contact thereby interrupting the circuit. In the design of current limiting contacts, it is important that when the contacts are closed there is sufficient contact pressure to assure minimal resistance to current flow but that the means for ensuring the requisite contact pressure does not inhibit the fast operation under fault conditions.
U.S. Pat. No. 4,409,573, which issued on Oct. 11, 1983 to Bernard DiMarco and Andrew J. Kralik, discloses a circuit breaker with a current limiting feature provided. The current limiting contacts blow open in response to fault current and are latched in the open position. The breaker is then reset by use of the operating handle. While this breaker achieves a current limiting effect, a higher current rating can be achieved by using blow open contacts of the current limiting type in series with this breaker. This configuration is disclosed in U.S. Pat. No. 4,458,224, which issued on July 3, 1984 to Bernard DiMarco and Andrew J. Kralik. In this embodiment, current limiting blow open contacts are placed in series with the circuit breaker. The blow open contacts are configured to reclose automatically by the action of biasing springs which also function to give the required closed contact pressure. It is apparent that the blow open force is a function of the current magnitude and the length of the parallel conducting paths which create the blow open force. The blow open force in this configuration is thus limited by the physical requirements of the circuit breaker enclosure. Accordingly, it would be appreciated that it would be highly desirable to provide increased blow open force for more rapid separation of the contacts due to a fault without increasing the physical dimensions of the circuit breaker enclosure.
U.S. Pat. No. 3,991,391, which issued on Nov. 9, 1976 to John A. Wafer, and U.S. Pat. No. 4,132,968, which issued on Jan. 2, 1979 to Walter W. Lane, disclose a current limiting circuit breaker which has a slot motor magnetic drive device. In this construction, the threshold level of overload current which produces current limiting action is raised, while the degree of current limiting action during high overload currents is maintained by placing a thin saturable magnetic steel plate across the open end of the slot motor magnetic drive device. During over current conditions below the threshold value, the plate shunts most of the magnetic flux and prevents production of magnetodynamic force upon the contact arm. Above the threshold level, the over current generates magnetic flux sufficient to saturate the plate and force additional flux into the air gap where the flux interacts with the contact arm to drive the contact arm into the slot and produce current limiting action in a normal manner. This configuration changes the normal response to a low level fault which the normal circuit breaker mechanism can handle and thereby limits the over current response of the current limiting contacts.
U.S. Pat. No. 4,001,738, which issued Jan. 4, 1977, to Claude Terracol and Pierre Schueller, discloses a circuit interrupter having an electromagnetic repulsion device. In this configuration, a circuit interrupter has a magnetic circuit energized by the current flowing through the interrupter and an induction plate that is movable with the movable contact of the interrupter. The abrupt rising of a fault current induces secondary currents in the induction plate which is located in the air gap of the magnetic circuit as long as the interrupter is in the closed circuit position. The secondary currents tend to expel the induction plate from the air gap thereby moving the movable contact vigorously away from the magnetic circuit. This increases the repulsing forces for a given current thereby ensuring fast opening operation. An alternate embodiment discloses contacts which form a two-loop current path. That is, a path in which current enters one conductor, flowing in a first direction, then flows through the movable contact in the opposite direction and then flows through the second stationary conductor in the first direction. This two-loop configuration effectively doubles the magnetic repulsion force. U.S. Pat. No. 4,118,681, which issued Oct. 3, 1978 to Jean Pierre Nebon and Robert Morel also discloses a circuit breaker having a two-loop blow off configuration. This patent also discloses a retarding member which is mechanically linked to the movable contact assembly to delay the reclosing of the contact and to prevent a reclosing before tripping of the circuit breaker. While the circuit breakers disclosed offer fast operation in response to a high level fault condition, there is still needed a circuit breaker which opens quickly and cleanly in response to a low level fault condition. Accordingly, it will be appreciated that it would be highly desirable to provide a circuit breaker contact structure which facilitates a fast response to low level fault conditions.
It will now be understood that it would be highly desirable to provide a current limiting circuit breaker which develops the maximum magnetic repulsive forces in a small amount of space and which opens and closes cleanly without undue hesitation.
It is an object of the present invention to provide a stationary contact assembly with a means to repel the movable contact assembly with double the normal blow-off force.
Still another object of the present invention is to provide a stationary contact assembly which is very compact yet which is not hindered by electrical interaction among the parts.